A class of the 9,9-dioctylfluorene-alt-5,7-bis(thiophen-2-yl)-2,3-biphenylthieno [3,4-b]pyrazine copolymeric derivatives (PFO-3ThPz-D) attaching additional donor (D) units in the pendant phenyl ring with a D-A D structure was synthesized and investigated, where the additional D unit is a substituent group of fluorene, carbazole, and triphenylamine (Tpa). Their photovoltaic properties were significantly tuned by these pending donor units. Among these copolymers, the PFO-3ThPz-Tpa exhibited the best photovoltaic properties in the bulk heterojunction polymeric solar cells (BHJ-PSC). The maximum power conversion efficiency (PCE) of 2.09% and the highest circuit current density (Jsc) of 7.91 mA/cm2 were obtained in the cell using a blend of PFO-3ThPz-Tpa and PC60BM (1:3, w/w) as active layer, which are 2.5 and 1.8 times higher than those corresponding levels in the other cell using the parent PFO-3ThPz-Ph copolymer instead of PFO-3ThPz-Tpa as donor, respectively. While PC60BM was replaced by PC70BM, the PFO-3ThPz-Tpa-based BHJ-PSC exhibited better photovoltaic properties with PCE of 3.08% and Jsc of 10.3 mA/cm2. This work demonstrated that attaching donor units into the D-A-based copolymeric side-chain is a simple and effective method to improve the photovoltaic properties for the resulting copolymers. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012